There is nothing in the usb port. I don't know about the WiFi band. Everything was set up by Verizon.This has been reported several time on the forum. I assume both the SP3 and SP4 are on exactly the same build and Marvell driver?
That has only happened to me once and when I tapped the BT icon twice it reconnected. Do you have anything connected to the USB port and what WiFi band are you using?